1. Field of the Invention
This invention relates to an image forming apparatus for transferring images on an image bearing member to a transfer medium born on a transfer medium bearing member to thereby form an image.
2. Related Background Art
The electrophotographic electrostatic recording system has often been used in color image forming apparatuses. As shown in FIG. 9 of the accompanying drawings, a color image forming apparatus is provided with an image bearing member 2 having a photosensitive medium on the surface thereof, an initial charger 16 for uniformly charging the image bearing member 2, an exposure device 13 having a light source such as a laser and transmitting a light signal and writing an electrostatic latent image onto the image bearing member 2, a developing device 6 containing therein color developers such as yellow (Y), magenta (M), cyan (C) and black (K) developers and developing the electrostatic latent image formed on the image bearing member 2 to thereby visualize it as a toner image, a transfer member 101 for holding a supplied transfer medium 14 by electrostatic absorption and by a gripper 11, repetitively conveying it to a transfer station opposed to the image bearing member 2 and transferring the toner image of each color formed on the image bearing member 2 onto the transfer medium 14, a cleaner 12 for removing any developer remaining on the image bearing member 2 after the transfer of the toner image of each color, and a fixating device 9 for fixating the toner image transferred onto the transfer medium 14.
The transfer member 101, as shown in FIG. 10A of the accompanying drawings, assumes a construction in which only a portion of the outer peripheral surface of a hollow drum housing 101c is left and the other transfer area is cut away and a flexible sheet 101a of a thickness of 30 to 250 .mu.m typified by polyethylene terephthalate (PET), polyvinylidene fluoride (PVdF), ethylene fluoride propylene copolymer (FEP), polycarbonate, polyuethane or the like is extended over the cut-away portion.
Inside and outside the transfer member 101, along the direction of rotation thereof, there is disposed an absorption roller 3 for electrostatically absorbing the transfer medium 14 onto the flexible sheet 101a, an absorption charger 5 opposed thereto with the flexible sheet 101a interposed therebetween, a transfer charger 4 for causing the toner image formed on the image bearing member 2 to be transferred onto the transfer medium 14, a separation charger 7 for separating the transfer medium 14 electrostatically absorbed onto the flexible sheet 101a therefrom, a separation pawl 15 which is an auxiliary separation means, and sheet electricity removal chargers 8 for initializing the potential of the flexible sheet 101a.
The image forming process of the above-described color image forming apparatus will now be described. The image bearing member 2 is first charged by an initial charger 16, and then the image bearing member 2 is exposed by a light signal based on an image signal of a first color, e.g. a magenta component color, from the exposure device 13 to thereby form an electrostatic latent image, which is developed by the developing device 6 containing the magenta developer therein, and is visualized as a magenta toner image. In parallel with this developing step, the supplied transfer medium 14 is rotated and twined on the transfer member 101 with its leading end gripped by the gripper 11 and the absorption roller 3 bears against the transfer medium 14 and nips it between itself and the surface of the transfer member 101. The transfer medium 14 is then electrostatically absorbed onto the surface of the transfer member 101 by charges imparted to the back of the flexible sheet 101a by the absorption charger 5, and the transfer medium 14 is held on the transfer member 101. The transfer medium 14 thus held on the transfer member 101 is conveyed to the transfer station opposed to the image bearing member 2 with the rotation of the transfer member 101, and the toner image formed on the image bearing member 2 is transferred onto the transfer medium 14 by the transfer charger 4 disposed in opposed relationship with the image bearing member 2.
After the termination of this transferring step of the first color, if the image bearing member 2 has any developer remaining thereon, the developer is removed by the cleaner 12 and an electrostatic latent image is again formed based on an image signal of a second color by charging and exposure, and the latent image is developed by the developing device 6 containing a developer of the second color, e.g. cyan, therein and is visualized as a cyan toner image. This cyan toner image is superposed on the magenta toner image and transferred onto the transfer medium 14 on the transfer member 101 onto which the magenta toner image has been previously transferred. Steps similar to what has been described above are also effected on third and fourth colors, e.g. yellow and black, whereby on the transfer medium 14, there is formed a color image comprising toner images of four colors superposed one upon another.
After the termination of the transfer of the toner images of four colors, the transfer medium 14 is conveyed to the location at which the inside and outside separation chargers 7 are disposed, by the rotation of the transfer medium 101, and the electrostatic absorption force between the transfer medium 14 and the flexible sheet 101a is removed by the separation chargers 7, whereafter the transfer medium 14 is separated from the transfer member 101 through the separation pawl 15. The thus separated transfer medium 14 is directed to the fixating device 9, where the mixing of the toner images of four colors and the fixation thereof onto the transfer medium 14 are effected. After the separation of the transfer medium, the transfer member 101 has its electricity removed by the sheet electricity removal chargers 8 and is electrically initialized, whereafter any developer adhering to the surface of the transfer member 101 are removed by a transfer member cleaner 10.
While in the foregoing, description has been made of the image formation by the color image forming apparatus using the transfer member 101 of the cut-away drum type, it has also been proposed to effect image formation by the use of a transfer member 1 having no cut-away portion as shown in FIG. 10B of the accompanying drawings. This transfer member 1 has its drum housing 1c formed into an electrically conductive cylinder free of a cut-away portion, and the drum housing 1c is covered with an electrically conductive elastic layer 1b formed of an electrically conductive foamed material such as urethane rubber, CR rubber, EPDM rubber or silicone rubber, and a flexible sheet 1a similar to the aforedescribed flexible sheet 101a is further superposed on the surface of the electrically conductive elastic layer 1b, and transfer can be accomplished by transfer bias being applied to the drum housing 1c.
The transfer member 1 free of a cut-away portion as described above has the advantage that its interior can be simplified as compared with the transfer member 101 of the cut-away drum type of FIG. 10A and thus the cost thereof can be reduced. In addition, the entire flexible sheet 1a is adapted to be supported from the inside thereof and therefore, the deformation and breakage of the flexible sheet 1a which are problems peculiar to the cut-away structure of FIG. 10A can be described to thereby extend the service life of the flexible sheet 1a. From such a point, attention is now paid to a color image forming apparatus using the transfer member 1 free of a cut-away portion (a transfer member of the bias drum type).
However, when the transfer member 1 of the bias drum type is used and color images are to be continuously formed on a plurality of transfer mediums 14 by an image formation starting signal being inputted once, there has been the tendency that color shift remarkably occurs to the color image formed on the first transfer medium and color shift does not occur to the second and subsequent transfer mediums. Further, there has been seen the tendency that on the first transfer medium, the image of the first color deviates relative to the images of the second, third and fourth colors.